Don't make it shit, please don't make it shit.
I've got a Nextcloud calendar linked through my NAS updating three LAN based devices, PC, tablet and phone, Thunderbird calendar on the PC works really well. As an email client its a brilliant alternative to Outlook for my purposes.
Hopefully the project will continue successfully, I'm planning to move my domains completely away from a web based email client and thunderbird seems to be the only contender so far. Really dislike web interfaces and I'm not so sure about web technology and how that would be used as an mail client interface. Would security be an issue?
Support the idea of bringing it into Open or Libre Office, I think mail client integration would be a useful facility.